1. Field of the Invention
The present invention relates to semiconductor light emitting devices and, more particularly, to quantum well semiconductor light emitting devices having improved carrier injection efficiency and power efficiency.
2. Background of the Invention
The emission wavelength of the semiconductor light emitting devices such as lasers developed to date is mainly in the near infrared (IR). This is limited by the band-gap of the semiconductor material in the active region where the stimulated recombination of electrons and holes across the band-gap results in the emission of electromagnetic radiation. Longer wavelength ( greater than 2 xcexcm) semiconductor light sources are demanded by many military and civilian applications such as free-space communications, medical diagnostics, atmospheric pollution monitoring, industrial process control, IR countermeasures, and IR radar for aircraft and automobiles. There have been many attempts at developing long wavelength IR sources by employing intersubband transitions in artificial quantum well (QW) semiconductor heterostructures since an original proposal by Kazarinov et al. in Soviet Phys. Semicond. Vol. 5 (4), 1971. The wavelength of such an IR source due to intersubband transition is not determined by the band-gap, but is instead determined by the smaller energy separation of conduction subbands arising from quantum confinement in QW heterostructures made from relatively wide band-gap semiconductor materials. Therefore, the emission wavelength can be tailored over a wide spectral range from mid-IR to sub-millimeter by merely changing QW layer thickness. One recent development in such an intersubband light emitting device was reported by Faist et al. in Science, Vol. 264, pp. 553-556, Apr. 22, 1994, who demonstrated a quantum cascade (QC) laser. A distinct feature of the QC laser is that each electron is reused with the capability of generating an additional photon as it cascades down each step of the energy staircase. Alternatively, interband cascade (IC) lasers, utilizing optical transitions between the conduction and the valence bands in a staircase of Sb-based type-II QW structures as originally proposed by Yang in Superlattices and Microstructures, Vol. 17, pp. 77-83, 1995 and U.S. Pat. No. 5,588,015 (which is incorporated by reference herein in its entirety), represent another new class of semiconductor mid-IR light sources. The IC lasers have recently been demonstrated with encouraging results. See, for example, by Yang et al., xe2x80x9cHigh power mid-infrared interband cascade lasers based on type-II quantum wellsxe2x80x9d published in Applied Physics Letters, Vol. 71, pp. 2409-2411, 1997. These lasers based on cascade configurations distinguish from the conventional bipolar semiconductor diode lasers in terms of equivalent circuit models. See, for instance, discussions in the paper published in J. Applied Physics, Vol. 79, pp. 8197-8203, 1996 by Yang et al. For a conventional bipolar diode laser consisting of several QWs in the active region for larger optical gain (or output power) and better optical confinement, each of the QWs can be viewed as light-emission units connected in parallel. On the other hand, the light-emission units in a cascade laser structure are connected in series. Ideally, as a consequence, current is traded for voltage in cascade laser structures. This means that the minimal bias voltage V0 and minimal injection current I0 required for a desired output power Po=I0V0 at a photon energy Ep=hv are related to the cascade stages Nc (equal to one for noncascade configurations) as expressed by
V0=Ncxc2x7Vp,xe2x80x83xe2x80x83(1)
I0=Po/(NcVp),xe2x80x83xe2x80x83(2)
where Vp=Ep/e. In practice, the applied bias voltage V and the total injection current I would not exactly follow the relationship as indicated by Eqs. (1) and (2) because there is always an excess voltage xcex94V and an excess current xcex94I in devices. How the excess current and voltage affect the laser performance can be evaluated in terms of power conversion (wall-plug) efficiency xcex7=Po/Pin. Expressing the input power Pin as,
Pin=IV=(I0+xcex94I)(V0+xcex94V),xe2x80x83xe2x80x83(3)
we can obtain                               η                      -            1                          =                  1          +                                    Δ              ⁢                              xe2x80x83                            ⁢              I                                      I              0                                +                                    Δ              ⁢                              xe2x80x83                            ⁢              V                                      V              0                                +                                                    Δ                ⁢                                  xe2x80x83                                ⁢                I                                            I                0                                      ·                                                            Δ                  ⁢                                      xe2x80x83                                    ⁢                  V                                                  V                  0                                            .                                                          (        4        )            
In an ideal case, xcex94I=xcex94V=0, and xcex7=1; so no power is wasted. In practical cases where there are additional voltage drops due to parasitic resistances, leakage currents, and nonradiative processes, which result in non-negligible excess voltage xcex94V and excess current xcex94I, there is always dissipated power PD=Pinxe2x88x92Po=Pin(1xe2x88x92xcex7). The resulting heat is detrimental to the operation of the device and limits the device performance. Therefore, it is especially important to maximize power efficiency. How this is done depends on the laser structure, approach, and materials. From Eq. (4), one finds that power efficiency is completely determined by the normalized excess current xcex94I/I0 and voltage xcex94V/V0 in equal weight, which are more appropriate than xcex94I and xcex94V for describing device performance. As discussed by Yang, xe2x80x9cMid-infrared interband cascade lasers based on type-II heterostructures,xe2x80x9d Microelectronics Journal, Vol. 30, No. 10, pp. 1043-1056 (1999) (attached hereto in the Appendix) and implied in Eq. (2), the normalized excess current xcex94I/I0 could be relatively large in a cascade structure for mid-IR lasers with many cascade stages, though its xcex94V/V0 can be small. One the other hand, the normalized excess current xcex94I/I0 could be relatively small in a conventional parallel structure for mid-IR lasers, but with a large xcex94V/V0. Therefore, power efficiency could be practically limited due to either relatively large xcex94I/I0 or xcex94V/V0 in both configurations for mid-IR lasers. Quantum well semiconductor superlattice structures are also discussed in R. Q. Yang, Chapter 2 in xe2x80x9cLONG WAVELENGTH INFRARED EMITTER BASED ON QUANTUM WELLS AND SUPERLATTICES,xe2x80x9d (M. Helm, ed., Gordon and Breach Science), entitled xe2x80x9cNovel Concepts and Structures for Infrared Lasersxe2x80x9d (attached hereto in the Appendix). Interband cascade lasers are discussed in R. Q. Yang et al., xe2x80x9cInterband cascade lasers: progress and challenges,xe2x80x9d Physica E, pp. 69-75 (2000), attached hereto in the Appendix.
The present invention is a semiconductor light emitting device comprising a multilayer semiconductor structure having a plurality of essentially identical active regions. The active regions are separated from their adjoining active regions by an injection region that serves as the collector for the preceding active region and as the emitter for the following active region. Carriers undergo interband transitions of energy states between the conduction band and the valence band in said active regions, resulting in light emission. Each of said active regions comprises multiple quantum well regions or finite superlattice regions to improve carrier injection efficiency and enhance optical gain without using a large number of cascade stages. This can reduce the operating voltage and increase the power efficiency.
The injection region comprises n-type and p-type semiconductor regions to form a type-II tunnel junction. In such a way, carriers are facilitated through interband tunneling for sequential photon emission under an appropriate bias, leading to the realization of an interband cascade configuration, which further improves the device performance.
The light emitting devices of the present invention utilize interband transitions of energy states between the conduction and valence bands in multiple quantum wells or superlattices, which differ from the intersubband or interminiband quantum cascade lasers as described by Capasso, et al. in U.S. Pat. Nos. 5,457,709 and 5,936,989. Compared to the structures disclosed in U.S. Pat. No. 5,588,015, the multiple quantum well (or superlattice) active region contained in each cascade stage in the present invention can have significant optical gain despite using a lesser number of cascade stages, leading to improved power efficiency.